The present invention relates generally to a fuel injection system having at least one hydraulically actuated fuel injector and, more particularly to controlling a supply of high pressure actuating fluid to the injector.
In a fuel system having hydraulically actuated electronically controlled unit injectors, such as HEUI injectors available from Caterpillar Inc., high pressure hydraulic actuating fluid drives a plunger to pressurize fuel and thereby inject high pressure fuel from a nozzle. An electronic activator, such as a solenoid, or a piezo-electric device, controls when the high pressure actuating fluid is exposed to the plunger. The amount of fuel injected is controlled by adjusting the duration the electronic actuator is xe2x80x9conxe2x80x9d.
The viscosity of the actuating fluid effects both the amount of fuel delivered by the injector, and when the fuel pressurization process begins. For example, at cold temperatures the actuating fluid is thicker (more viscous) than at warm temperatures. Therefore, when an electrical signal is delivered to an electronic actuator, the fluid flows into the injector at a relatively slow rate, to drive the plunger. With the actuating fluid moving at a relatively slow rate, there is an increased delay before the injector begins delivering fuel. Furthermore, when the electronic actuator is turned off to stop delivery of the fuel, the reduced flow rate of the actuating fluid results in less than the intended amount of fuel being injected. Hence, with a high viscosity actuating fluid as seen at cold starting temperatures as compared to higher temperature operating conditions, the fuel injection event occurs later than intended due to the slower delivery rate of the actuating fluid. Under these conditions, overall engine performance may be adversely effected, resulting in incomplete combustion, low power, white smoke, unused particulate matter, and NOx.
The viscosity of the actuating fluid is a function of the fluid type, the temperature of the fluid, and the shear rate of the fluid in the hydraulic circuit. In an operating engine, neither the type of fluid, the shear rate, nor the temperature is fixed. The fuel system may use a variety of actuation fluids. For example, a more viscous SAE 15W40 engine oil or a less viscous0W20 engine oil may be used. Also the fuel system operates over a wide range of temperatures, e.g., xe2x88x9245xc2x0 C. through 120xc2x0 C.
The viscosity of the actuating fluid changes with a change in shear rate at a given temperature. During cranking at either hot or cold starting conditions, the viscosity of the actuating fluid is temporarily lowered when the flow rate of the fluid is increased and the well sheared actuating fluid enters the actuating fluid circuit. The temporary viscosity loss can not be detected by the engine governor or the vehicle operator. The sudden, temporary loss of viscosity produces a sudden increase in fuel delivery, which in turn creates a rapid change in engine speed.
The reduction in fuel delivery and delays in timing increase as the viscosity of the actuating fluid increases. If the changes in shear rate, which temporarily change the viscosity, are not accounted for, the fuel delivery and timing may be incorrect making it difficult to start and run the engine especially at high viscosities encountered at cold temperatures. If the fuel delivery is too small, or is not delivered at the proper time, the engine may not start or be underpowered. If the fuel delivery is too large the engine structural capabilities may be exceeded, excessive smoke may be produced, and misfire may occur.
The present invention is directed to overcoming one or more of the problems identified above.
In one aspect of the present invention, a method or operating a fuel injection system including at least one hydraulically actuated fuel injector fluidly connected with a source of high pressure hydraulic actuation fluid is disclosed. The method includes the steps of determining the viscosity of the actuation fluid, the rate of change of the viscosity, and controlling the supply of actuation fluid to the fuel injector based, at least in part, on the determined viscosity of the actuation fluid.
In another aspect of the present invention a fuel injection system is disclosed. The fuel injection system includes at least one hydraulically actuated fuel injector fluidly connected with the source of high pressure actuation fluid, a viscosity sensor for determining the viscosity of the high pressure hydraulic actuation fluid, and a controller in communication with the hydraulically actuated fuel injector being adapted to determine the rate in change of the viscosity of the high pressure actuating fluid, and determining a fuel injection command signal in response to the rate of change of the viscosity of the high pressure hydraulic actuation fluid.